cmd/internal/gc: unembed Param field

cmd/internal/gc: unembed Param field

This is an automated follow-up to CL 10210.
It was generated with a combination of eg and gofmt -r.

No functional changes. Passes toolstash -cmp.

Change-Id: I35f5897948a270b472d8cf80612071b4b29e9a2b
Reviewed-on: https://go-review.googlesource.com/10253
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>